With the increasing development of integrated circuit and decreasing size of field effect transistors, stress technology is introduced into semiconductor fabrication to change the lattice structures of the channel, so as to improve the carrier mobility in the channel. From the present research, tensile stress applied on the channel improves the electron mobility, while compressive stress applied on the channel improves the hole mobility. Embedded SiGe technology, which embeds SiGe in the source/drain regions of the PMOS devices to apply compressive stress on the channel region, is widely used to increase the performance of PMOS devices.
In the embedded SiGe process, the increase of Ge content in the SiGe layer will raise the stress to the channel, which improves the performance of the PMOS devices. However, due to the rising Ge content difference between the Si substrate and SiGe layer, lattice mismatch also increases, which results in the dislocation between the Si substrate and SiGe layer and the degradation of the device performance.
Moreover, due to the selectivity of epitaxial growth (the epitaxial growth rate GR on different crystal orientations ranks as GR<100>>GR<110>>GR<111>), a <111> crystal plane is prone to be formed on opposing sides of the SiGe epitaxial layer when the SiGe epitaxial layer is higher than the substrate surface in SRAM regions. However, the <111> crystal plane resist to the growth of a subsequent cap layer, resulting in poor uniformity of the cap layer in the SRAM regions (the cap layer growing on the <111> crystal plane has a very small thickness). On the other hand, since the SiGe epitaxial layer with high Ge content is incapable to react with the metallic Ni to form metal silicide such as NiSi or NiGeSi, bad contact between the SiGe epitaxial layer and subsequent contact holes may occur to cause problems such as electric leakage, rising or uncontrollable resistance.
In view of the drawbacks with the prior art, there exists a need to develop a new method which increases the content of Ge in the SiGe source/drain regions, reduces or eliminates dislocations, improves the profile of the cap layer, and facilitates the formation of the metal silicide (NiSi) at the same time.